This invention relates to the field of non-invasive measurements of functions of biological tissues.
Structural damage in living tissues is always accompanied by functional deficit. However, the converse is not necessarily true. That is, functional deficit may precede irreversible structural damage for many years, in numerous disease states, and may therefore serve as a diagnostic indicator of early disease and a prognostic indicator of disease progression. For this reason, increasing attention has focused on functional imaging of tissues, in situ, in humans.
PET scanning provides striking images of functional change; however, its resolution is crude and its implementation expensive.
To provide high-resolution imaging of the functional state or metabolic rates of tissues non-invasively and in situ, optical techniques must be employed. The present invention provides such an optical technique.
The functional status of bodily tissues is stoichiometrically related to tissue metabolism through the well-established mechanism of respiratory control. In particular, in the process of electron transfer from substrates such as glucose and pyruvate to molecular oxygen, the oxidation of the flavin adenine dinucleotide FADH2 to FAD produces 2 molecules of adenosine triphosphate (ATP), while the oxidation of the nicotinamide adenine dinucleotide NADH to NAD yields 3 ATPs. ATP in turn is used to power processes within living cells that support function within all living cells. Therefore, the conversions of these two nucleotides may be used to monitor cellular function and serve as sensitive indicators of cellular and tissue health.
Both flavin and nicotinamide dinucleotides possess fluorescence properties and lifetimes that change with cellular function and both are endogenous fluorophors that are found within mitochondria as well as in enzymes within other cellular compartments in tissues. Prior measurements of the fluorescence intensity of either or both, or of the ratio of fluorescence intensities of these molecules, have been used in research studies by Chance and his colleagues to explore metabolism in a research setting. However, these approaches have never successfully been extended to the clinical setting because they require calibration of the system by bringing the tissue to a uniform oxygen partial pressure of 0 mm Hg by breathing an animal on 100% nitrogen. This obviously cannot be done in humans without causing irreversible cell death.
The need to calibrate fluorescence intensity measurements arises from the photobleaching of fluorophors during exposure to excitation light. Moreover, fluorescence intensity is influenced by other factors that have no relation to metabolism, such as absorption of excitation and emission light by intervening tissues. To overcome these deficiencies, some have tried to take the ratio of emissions from NADH and FAD, but the results have not been satisfactory because the rates of photobleaching of these molecules are different, thus precluding a clinically useful tool.
In theory, the problems associated with fluorescence intensity measurements may be overcome by measuring fluorescence lifetime, namely the decay constant of fluorescence emission following pulse excitation. However, the fluorescence lifetimes of interest would require the use of femtosecond laser pulses. Even with the use of photon counting photomultipliers, such a technique would require, for adequate signal-to-noise ratio, an excitation energy that would destroy tissue.
U.S. Pat. No. 5,626,134, the disclosure of which is incorporated by reference herein, describes a novel procedure for the steady-state measurement of fluorescence lifetime, based upon the measurement of fluorescence anisotropy. The disclosed general methodology overcomes the deficiencies of time-resolved measurements of fluorescence lifetime in both in vivo and in vitro applications.
The present invention provides a modified technique which can be applied to the non-invasive measurement of the steady-state fluorescence anisotropies of flavin and nicotinamide dinucleotides within bodily tissues. As will be shown in detail below, the present invention uses measurements of steady-state fluorescence anisotropy to reveal tissue functional and metabolic status, and changes in function and metabolism that accompany disease states. Moreover, and of singular importance, steady-state fluorescence anisotropy is employed in the present invention to reveal numerous aspects of metabolically-induced changes in these endogenous nucleotides, namely, fluorescence lifetime changes that occur during function and metabolic change as well as conformational changes and changes from the unbound to bound form of these nucleotides that also take place during metabolism and function within bodily tissues.
In the present invention, the application of steady-state fluorescence anisotropy measurement is a more encompassing and broader methodology for probing the functional and metabolic state of a bodily tissue in situ by non-invasive methods in health and disease. The method and apparatus thereby provide the first safe, sensitive, calibration-free optical methodology for the non-invasive measurement of metabolic rate and functional status of tissues in 2- and 3-dimensional space.
Ideally, a noninvasive optical approach that reveals function by imaging metabolic changes should yield signals that are quantitatively traceable to function based upon the stoichiometric relationship between function and metabolism that is imposed by respiratory control. Redox fluorometry is one well-established approach that fulfills this criterion. For this reason, redox fluorometry employing the intrinsic fluorescence of reduced pyridine nucleotides and oxidized flavoproteins has long been employed to assess cellular energy metabolism. However, such intensity-based methods are severely limited due to photobleaching, inner filter effects and the difficulties associated with isolating contributions from these metabolically relevant fluorophores from the background autofluorescence of other endogenous fluorophores. Inner filter effects are present in all tissues. Although measurements of fluorescence lifetime can, to some extent, overcome the limitations of intensity-based measurements, the information gleaned from such measurements alone is limited, and ultra fast lifetimes require the use of high energy laser pulses that may be damaging to fragile tissues.
The present invention includes a novel method of steady-state flavoprotein fluorescence anisotropy imaging (metabolic mapping) that overcomes the deficiencies of intensity- or lifetime-based imaging while retaining the essential quantitative coupling of metabolism to function. The steady-state fluorescence anisotropy (A) of a distinct molecular species undergoing isotropic rotational diffusion is related to the excited state lifetime τ and the rotational correlation time φ by the following equation:
                                          A            o                                A            _                          =                              1            +                          τ              ϕ                                =                      1            +            σ                                              (        1        )            where Ao is a limiting value (in the absence of rotation) given by the relative orientation of the absorption and emission dipole transition moments, and σ is the ratio τ/φ.
From this equation it follows that fluorescence anisotropy is a parameter with the capability of revealing changes in both orientation distribution and excited state lifetimes with great sensitivity. Such function-induced metabolic changes could arise from restrictions to diffusional motion, complex formation and molecular proximity manifested by hetero- or homo-energy transfer. Moreover, fluorescence anisotropy and lifetime are intrinsic parameters, unlike the intensity signals used to compute them, and are therefore insensitive to light path and geometry. When measurements are restricted to a single intrinsic fluorophore, the effects of photobleaching are also eliminated. All of these advantages contribute to the capability of making reliable comparisons over time in the same tissue in situ and between different living tissues.
Fluorescence anisotropy is independent of fluorophore concentration, and therefore, unlike fluorescence intensity-based imaging or structural technologies, its sensitivity is independent of the thickness of the tissue being probed. Of especial importance is the large safety margin of the method of the present invention that allows it to be employed to probe cellular bioenergetics in living tissues in humans and the ability to bandpass and notch filter anisotropy values provides the opportunity to reject contributions from other endogenous fluorophores.
Flavoprotein (FP) fluorescence can be excited by longer wavelength, lower energy light, is more resistant to photobleaching than pyridine nucleotides (NADH or NADPH) and is almost singularly associated with mitochondria (Koke et al, “Sensitivity of flavoprotein fluorescence to oxidative state in single isolated heart cells”, Cytobios (1981), vol. 32, p. 139-145; Scholz et al, “Flavin and pyridine nucleotide oxidation-reduction changes in perfused rat liver”, J. Biol. Chem. (1969), vol. 244, p. 2317-2324). By restricting measurements to FP steady-state fluorescence anisotropy, the risk of damage to tissues by high energy laser pulses is obviated by the use of lower energy light distributed over durations that are long relative to the excited state lifetimes. Of the numerous enzymes endowed with flavin cofactors, it has been previously shown that lipoamide dehydrogenase (LipDH) dominates the fluorescence signal with lesser contributions from electron transfer flavoprotein (ETF). LipDH serves as a direct probe of cellular metabolism and function because its FAD cofactor is in direct equilibrium with the mitochondrial NAD+/NADH pool, while the redox state of ETF is indirectly affected by the NAD+/NADH ratio within mitochondria.
It should also be noted that time-resolved fluorescence anisotropy measurement requires the use of ultra-fast laser pulses that would similarly be destructive to tissues. For this reason, steady-state fluorescence anisotropy determination is employed in the current invention. Furthermore the measurement of flavin dinucleotide fluorescence anisotropy is a preferred embodiment of the technology because longer excitation wavelengths and thus lower excitation energies may be employed, thereby ensuring the safety of its use in delicate tissues such as in the non-invasive imaging of the functional status of the human retina, in situ, in the eye.
Although, the methodology disclosed herein is directed toward non-invasive measurement of the fluorescence anisotropies of flavin and nicotinamide dinucleotides, and thereby the metabolic and functional status of tissues, it will be apparent to one skilled in the art that this is a general methodology that may be applied to all endogenous fluorophors whose steady-state fluorescence anisotropies may be influenced by disease processes.
The present invention also includes a method for extending the sensitivity of the above-described non-invasive method by measuring the steady-state fluorescence anisotropy of the tissue first in the resting state and subsequently in the stimulated state. Steady-state fluorescence anisotropy maps are acquired in these two states in 2- or 3-dimensional space and the anisotropy map in the resting state is subtracted point-by-point from that obtained in the stimulated state. The resultant fluorescence anisotropy map therefore reveals the capacity of the tissue to respond to stimulation. As such, the methodology has clear application to detection and prognosis of disease states wherein the magnitude of change to stimulation may be reduced and the spatial loci of functional and metabolic deficits identified.